Chronic joint pain/swelling, numbness, fatigue, migraines, mouth sores, swollen lymph nodes, weight loss, pain when breathing, nausea. lupus?

Or something else: Lupus and other connective tissue diseases can share these symptoms. Infection followed by malignancy are less likely but not impossible. Either way, you as a 25 year old female should see a rheumatologist, or alternatively internist if rheum is not readily available. Early treatment is advised.

Immune system: Difficult to say without thorough medical evaluation by a primary care physician. One thing that comes to mind to be ruled out is mono. Review with your doctor.